Decarburisation of chromium containing iron, cobalt or nickel based alloys

ABSTRACT

In a process for decarburising stainless steel by blowing the molten steel with a mixture of oxygen together with steam and/or ammonia, blockage of gas inlet is avoided by passing the mixture of oxygen together with steam and/or ammonia through the inner section of a tuyere comprising at least two coaxial pipes and passing through the outermost section of the tuyere a gas e.g. oxygen and steam or oxygen and nitrogen, which undergoes an exothermal reaction when it contacts the molten alloy.

nite States Erilrsson et al.

atent 4] DECARBURISATION F CHROMIUM CONTAINING IRON, COBALT 0R NICKELBASED ALLOYS,

[75] Inventors: Lars Anders Eriksson; Karl-Erik Oberg, both of Hagfors,Sweden [73] Assignee: Uddeholms Aktiebolag, Hagfors,

Sweden [22] Filed: Oct. 5, 1973 [21] Appl. No.: 403,854

[30] Foreign Application Priority Data Oct. 6, 1972' Sweden 12907/72[52] US. Cl 75/60, 75/59, 75/82 [51] Int. Cl. C2llc /32 [58] Field ofSearch 75/59, 60, 82

[56] References Cited UNITED STATES PATENTS 2,855,293 /1958 Savaro 75/601' Feb. 18,1975

3,706,549 12/1972 Knuppel /60 FOREIGN PATENTS OR APPLICATIONS 1,450,7187/1966 France 75/60 Primary Examiner-L. Dewayne Rutledge AssistantExaminerPeter D. Rosenberg Attorney, Agent, or FirmCushman, Darby &Cushmanv [5 7] ABSTRACT gas e.g. oxygen and steam or oxygen andnitrogen,

which undergoes an exothermal reactionwhen it contacts the molten alloy.

11 Claims,-l Drawing Figure i DECARBURISATION F CHROMIUM CONTAININGIRON, COBALT OR NICKEL BASED ALLOYS The present invention relates to animproved process for reducing the carbon content of an iron, cobalt ornickel based alloy containing carbon and at least percent chromium,particularly stainless steels, by introducing into the molten alloy,oxygen together with steam and/or ammonia which decompose in the meltyielding hydrogen. This hydrogen is able to influence the partialpressure of carbon monoxide (derived by oxidation of the carbon) in themelt and so control the extent of oxidation of chromium and otherreadily oxidisable components in the melt during the decarburisation.

The basic decarburisation process is described in detail in U.S.v Pat.application Ser. No. 295355 filed Oct. 10th 1972 by M. K. O. Johnssonand L. A. Ericksson. Briefly, this decarburisation process involves theuse of a mixture of oxygen with a high percentage of steam and/orammonia. During the main part of the decarburisation period the volumeratio oxygen: steam ammonia should preferably be less than 3 1 and arecommended value is between 1 l and l 10. This high content of steamand/or ammonia has a great cooling effect on the steel, the more sosince the gas mixture should be injected at a speed which exceeds thatof sound. As a result the tuyeres show a marked tendency to get blockedby solidification of the melt. The present invention provides a methodof decarburisation in accordance with the general procedure of the abovedescribed Patent Application but under conditions which reduce thetendency for the tuyeres to get blocked without giving rise,simultaneously, to new problems due to excessive energy development inthe tuyere area, which would entail an unacceptable rate of wear of thetuyeres and/or of the area in the proximity of these.

The present invention provides a method of'reducing the carbon contentof an iron, cobalt or nickel based alloy containing carbon and at least10 percent by weight chromium by introducing into the molten alloyoxygen and a hydrogen yielding gas which is steam or ammonia or mixturesthereof wherein a mixture of oxygen and the hydrogen yielding gas isintroduced into the molten alloy through an inner section of a tuyerecomprising at least two coaxial pipes, the amount of hydrogen yieldinggas being sufficient to bring about an endothermal reaction when thehydrogen yielding gas meets the molten alloy, and introducing throughthe outermost section of the tuyere a gas which undergoes an exothermalreaction when it meets the molten alloy.

The invention will now be described more in detail, with reference tothe accompanying drawing, which shows the principles for the design of atuyere for use in the invention.

In the figure, tuyere 1 consists of two concentric pipes, an inner pipe2 and an outer pipe 3. The pipes can, for example, be made of copper,carbon steel or stainless steel. The opening between the pipes isindicated by 4. Speed, flow and number of tuyeresvwilldetermine the sizeof the tuyeres. A suitable diameter for the inner pipe is 5 to 15 mm.and the width of opening 4 should be at least 0.5 mm but not more than 2mm. Tuyere 1 extends through the bottom 6 (or the wall) of the converteror other process vessel, containing the melt 5 in process ofdecarburization. Tuyere 1 has a contraction 7, which divides the tuyereinto two parts, namely a forward part8 and a rear part 9. Forward part 8has a narrower cross-section than rear part 9, but the ratio of thecross sectional areas of each of these two parts is equal. Thisarrangement brings about a pressure reduction in the delivery lines whenthe tuyere has undergone wear reaching the area of the contraction. Thislowered pressure can be used as an indication that the tuyere or thebottom of the converter needs replacing. In an embodiment of theinvention aiming at exploiting thisfacility, the lines connected to thetuyere are therefore connected to suitable regulating members, whichsense and register the pressure in the lines.

In order to ensure sufficient impulse when the gas mixture is injectedinto the melt, the injection speed should exceed the critical speed,i.e., that of sound. The total flow of gas through the tuyere or thetuyeres should be 0.5 to 5 standard cubic metres per metric ton of meltper minute (St.m /ton/min). Preferably 0.5 to 3 St.m /ton/min.

In implementing the method according to the invention the mixture ofoxygen with steam and/or ammonia is injected through the inner pipe 2.The steam and/or ammonia content of the mixture should be at least 25percent by volume, and in practice is usually 50 to 90 percent by volume(volume ratio oxygen steam and/or ammonia 1 1 1 10). Under thesecircumstances, the gas mixture used for the decarburisation has a strongcooling effect because of the large amount of thermal energy requiredfor the decomposition of the steam and/or ammonia. In order to reducethe tendency for the tuyere to become blocked by solidification of themelt, a gas mixture having an exothermal reaction with the melt in thearea of the orifice of the tuyere is injectedthrough opening 4 betweeninner pipe 2 and the outer pipe 3. The exothermah gas will normallycontain a large proportion of oxygen but the proportion of oxygen shouldnot be so high as to cause damage to the tuyere itself or to the partsadjacent to it as a result of excessive heat generation close to thetuyere orifice. Examples of suitable gas mixtures to be injected throughopening 4 are l. 40 O 60-20% steam 2. l0 50% 0 -50% argon 3. 5 30% O-70% N all percentages being by volume. Combinations of these gasmixtures are also possible. The rate of flow through opening 4,preferably amounts to less than 20 percent, and particularly 1015percent of total volume of gas flow through the tuyere.

The tuyere described above contains only two concentricpipes but itispossible to operate in accordance with the invention using a tuyerehaving 3 or more concentric pipes. In this case, the endothermic gasmixture is injected through one or more inner sections of the tuyere butnot through the outermost section while the exothermic gas mixture isinjected through-the outermost section. This arrangement is alsoconvenient when it is desired to inject ammonia separately from oxygenin which case oxygen is injected through one inner section, ammonia isinjected through a different inner section and the exothermic gasmixture is injected through the outermost section.

In the drawing, only one tuyere is shown but in practice, thedecarburisation will be carried out using several tuyeres of this type.

The type of alloys which can be treated in this invention and thegeneral details of the decarburisation and ancillary refining steps usedin this invention are the same as those disclosed in the patentapplication mentioned in the second paragraph of this specification andthose disclosures are incorporated in this specification bythis'reference.

We claim:

1. A method of reducing the carbon content of an iron, cobalt or nickelbased alloy containing carbon and at least percent by weight chromium byintroducing into the molten alloy oxygen and at least one hydrogenyielding gas selected from the group consisting of steam and ammoniawherein a first mixture consisting essentially of a minor proportion byvolume of oxygen and a major proportion by volume of the hydrogenyielding gas is introduced into the molten alloy through an innersection of a tuyere comprising at least two coaxial pipes and having atleast one inner section and an outermost section, the amount of hydrogenyielding gas being sufficient to bring about an endothermal reactionwhen the hydrogen yielding gas meets the molten alloy,

and introducing through the outermost section of the tuyere a secondmixture which is a gas which'undergoes an exother'mal reaction when itmeets the molten alloy and which second mixture consists essentially ofoxygen and a cooling gas.

2. A method according to claim 1 where the volume of gas flowing throughthe outermost section of the tuyere-is not more than 20 percent of thetotal volume of gas flowing through the tuyere.

3. A method according to claim 2 where the flow through the outermostsection is 10 percent by volume of the total flow.

4. A method according to claim 1 wherein the gas flowing through theoutermost section contains 40 percent by volume oxygen and 60 20 percentby volume steam.

5. A method according to claim 1 wherein the gas flowing through theoutermost section contains 5 30 percent by volume oxygen and 70 percentby volume nitrogen.

6. A method according to claim 1 wherein the gas flowing through theoutermost section contains a major amount by volume of nitrogen andminor amounts by volume of oxygen and steam.

7. A method according to claim 1 wherein at least one tuyere has acontraction in its cross sectional area at a position spaced apart fromthat end of the tuyere opening into the melt and at least one feed lineto the tuyere is provided with a pressure sensing device which canindicate, by recording pressure variation in the feed line, when thetuyere has been corroded away as far a the contraction.

8. A method according to claim 1 wherein'the alloy is a stainless steel.

9. A method according to claim 1 wherein the volume ratio of oxygen tosteam and/or ammonia in the gas mixture introduced through the innersection of the tuyere is l 1 to l 10.

10. A method according to claim 1 wherein the rate of flow of the gasesintroduced into-the molten alloy is 6.5 5 standard cubic metres permetric'ton per minute.

' 11. A method according to claim 1 wherein the cooling gas is at leastone selected from the group consisting 4 of steam, nitrogen and argon.

1. A METHOD OF REDUCING THE CARBON CONTENT OF AN IRON, COBALT OR NICKELBASED ALLOY CONTAINING CARBON AND AT LEAST 10 PERCENT BY WEIGHT CHROMIUMBY INTRODUCING INTO THE MOLTEN ALLOY OXYGEN AND AT LEAST ONE HYDROGENYIELDING GAS SELECTED FROM THE GROUP CONSISTING OF STEAM AND AMMONIAWHEREIN A FIRST MIXTURE CONSISTING ESSENTIALLY OF A MINOR PROPORTION BYVOLUME OF OXYGEN AND A MAJOR PROPORTION BY VOLUME OF THE HYDROGENYIELDING GAS IS INTRODUCED INTO THE MOLTEN ALLOY THROUGH AN INNERSECTION OF A TUYERE COMPRISING AT LEAST TWO COAXIAL PIPES AND HAVING ATLEAST ONE INNER SECTION AND AN OUTERMOST SECTION, THE AMOUNT OF HYDROGENYIELDING GAS BEING SUFFICIENT TO BRING ABOUT AN ENDOTHERMAL REACTIONWHEN THE HYDROGEN YIELDING GAS MEETS THE MOLTEN ALLOY, AND INTRODUCINGTHROUGH THE OUTERMOST SECTION OF THE TUYERE A SECOND MIXTURE WHICH IS AGAS WHICH UNDERGOES AN EXOTHERMAL REACTION WHEN IT MEETS THE MOLTENALLOY AND WHICH SECOND MIXTURE CONSISTS ESSENTIALLY OF OXYGEN AND ACOOLING GAS.
 2. A method according to claim 1 where the volume of gasflowing through the outermost section of the tuyere is not more than 20percent of the total volume of gas flowing through the tuyere.
 3. Amethod according to claim 2 where the flow through the outermost sectionis 10 - 15 percent by volume of the total flow.
 4. A method according toclaim 1 wherein the gas flowing through the outermost section contains40 - 80 percent by volume oxygen and 60 - 20 percent by volume steam. 5.A method according to claim 1 wherein the gas flowing through theoutermost section contains 5 - 30 percent by volume oxygen and 95 - 70percent by volume nitrogen.
 6. A method according to claim 1 wherein thegas flowing through the outermost section contains a major amount byvolume of nitrogen and minor amounts by volume of oxygen and steam.
 7. Amethod according to claim 1 wherein at least one tuyere has acontraction in its cross sectional area at a position spaced apart fromthat end of the tuyere opening into the melt and at least one feed lineto the tuyere is provided with a pressure sensing device which canindicate, by recording pressure variation in the feed line, when thetuyere has been corroded away as far a the contraction.
 8. A methodaccording to claim 1 wherein the alloy is a stainless steel.
 9. A methodaccording to claim 1 wherein the volume ratio of oxygen to steam and/orammonia in the gas mixture introduced through the inner section of thetuyere is 1 : 1 to 1 :
 10. 10. A method according to claim 1 wherein therate of flow of the gases introduced into the molten alloy is 6.5 - 5standard cubic metres per metric ton per minute.
 11. A method accordingto claim 1 wherein the cooling gas is at least one selected from thegroup consisting of steam, nitrogen and argon.